An existing case-control study of aplastic anemia in Thailand will be extended and modified to explore new hypotheses raised thus far and to provide more definitive results for previously borderline findings. New information will be collected and the data collection network will be reduced to a single hospital in Bangkok and eight hospitals in Khonkaen, a rural region in the northeast. Incident cases will be identified prospectively and reviewed by study hematologists to confirm the diagnoses. Suitable hospital controls (e.g., patients with trauma and other acute conditions) will be matched to cases for age, sex, and the time of hospitalization. Subjects will be interviewed to obtain information on occupational and chemical exposures, farming practices, sanitation, drug use, personal data, and relevant medical history. Blood samples will be collected from all cases and at least one control per case, and stored for evaluation of future hypotheses. A total of 195 cases and 780 controls will be enrolled. Hypotheses to be evaluated with the new data include the following: whether an association with grain farming that explains a substantial proportion of cases is due to soil exposure rather than a chemical agent; elucidation of an association with occupational exposure to solvents, with a focus on details of the specific solvents used; the possibility that poor sanitation increases the risk, suggested by associations with low socioeconomic status and hepatitis A; and a more detailed evaluation of occupation. The new subjects will be combined with the 284 cases and 1174 controls already enrolled to conduct a detailed investigation of specific agricultural pesticides, to explore an apparent lack of association with chloramphenicol, and to evaluate other borderline results concerning drugs (e.g., mebendazole). The new phase of data collection has already begun with support from other sources, which will cover 2.5 years; the proposed study will cover the two additional years of data collection and the analysis. The investigators state that this large study has proven to be feasible, valid, and informative, with ten papers already published or in press, and one submitted. They further state that completion of the project as proposed will maximize its value and be more efficient than a new study.